Phrenologists missing, in short, and that's the absolute truth (6)

I believe the answer is:

gospel

I'm a little stuck... Click here to teach me more about this clue!

'absolute truth' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for gospel that I've seen before include "Type of music" , "church music" , "Truth (religious)" , "Absolute truth" , "sacred text" .)

I've seen this clue in The Irish Times.

Want a hint initially instead of a full solution? Install my app